Missing symbols in the sansmath package

Question

When using the sansmath package with the Arev font, symbols like + and = fail to appear in the compiled document.

\documentclass{article}
\usepackage{fouriernc} % use the Arev font
\usepackage{sansmath}
\begin{document}
You can see this $+$ and this $=$.
But you can't see \sansmath this $+$ and this $=$.
\end{document}

If, however, I switch the order of \usepackage to include sansmath first and then fouriernc, for some reason you now can see the symbols, but I get two errors...

\documentclass{article}
\usepackage{sansmath}
\usepackage{fouriernc} % use the Arev font
\begin{document}
You can see this $+$ and this $=$.
Now you can see \sansmath this $+$ and this $=$, but there are 2 errors.
\end{document}

! LaTeX Error: Command `\mathrm' already defined.

! LaTeX Error: Command `\mathrm' already defined.

Answer 1

The package fouriernc doesn't use Arev fonts, but rather NewCentury Schoolbook and a modified version of the Fourier math fonts.

These fonts have different symbol positions and so they define + and = differently from normal: the normal mathcode of + is "202B (in hexadecimal form), while fouriernc uses "2280.

These cryptic codes say: + is an operation symbol (first digit 2), which comes from math family 0 (in the standard setting) or 2 (in the fouriernc settings). The symbol lives in font position "2B (standard setting) or "80 (fouriernc).

The package sansmath expects standard codes; in family 2 it uses cmsy10 which in position "80 has nothing (it's a seven bit font).

So the two packages are completely incompatible.